1. Field of the Invention
This invention relates to sound and navigation ranging (sonar) arrays employing a solid state preamplifier apparatus which provides improved signal to noise ratio performance, and reductions in cost, required space and power consumption.
2. Description of the Related Art
As is well known by those skilled in sonar technology, hydrophones are normally used in arrays in which a number of spaced elements are employed. Such arrays have several advantages over a single hydrophone, namely, greater sensitivity, directional properties, and better signal-to-noise ratio. (Robert J. Urick, Principles of Underwater Sound, (3rd ed.; New York, McGraw Hill Book Company, 1983), p. 32. Typically, each hydrophone in any array is followed by a separate preamplifier channel in which amplification and shading functions are performed. These shading functions, used to determine the shape of the sonar beam pattern, are well understood by those skilled in the art. The outputs of the preamplifier channels of the array are combined in beamforming apparatus to form one or more sonar beams. Since sonar arrays may consist of anywhere from two to a large number of hydrophones, economy of space, cost, and power usage are important factors in the design process. Because of the low power output of hydrophones, preamplifier voltage gains of the order of 40 dB have been used. However, the manner in which prior-art preamplifiers have been coupled to the analog beamforming apparatus has resulted in significant attenuation of the signal at the beamformer. Specifically, most preamplifiers have voltage-source outputs, whereas the beamforming apparatus has required current-source drive. To effect conversion of the preamplifier voltage source to a current source, a resistor whose resistance was high in comparison to the beamformer input impedance was used to couple each preamplifier output to the beamformer, a technique well-known to circuit designers. While effective for the described conversion, the technique results in significant undesired attenuation of the signals at the beamformer's inputs.
Two disadvantages result from the aforementioned undesired attenuation. First, subsequent amplification after the beamformer has been required to compensate for the attenuation. Second, the attenuated signals at the beamform have been at such a low level that pickup of even low levels of undesired electromagnetic interference (EMI) on the beamformer and its connecting leads has been sufficient to degrade the signal-to-noise (S/N) ratio. Shielding and other measures to reduce the EMI result in increased cost.
Simplified preamplifiers which have current-source outputs, with feedback stabilized gain, which therefore may be coupled directly to the beamforming apparatus without causing the aforementioned attenuation is not found in prior-art apparatus.